FreeBSD 5.1 -RELEASE: unknown option "MAXUSERS"

[Joe Warner]

Hi,
 
I'm running FreeBSD 5.1 -RELEASE and I'm getting the error in the subject 
after attempting to add a new option to my custom kernel. 
 
I'm trying to add "options USER_LDT" so I can install the new Nvidia drivers 
but when I issue:
 
 /usr/sbin/config CUSTOM_KERNEL 
 
 ..I get:
 
 custom_kernel: unknown option "MAXUSERS"
 
 If I issue:
 
 cd /usr/src
 
 make buildkernel KERNCONF=CUSTOM_KERNEL
 
 ..I get:
 
 make: don't know how to make buildkernel. Stop
 
I was able to create a custom kernel and compile it successfully after I 
installed 5.1 so what gives?
 
I've run cvsup a couple of times using the RELENG_5 tag in my supfile but it 
never pulls down any source and I assume this is because there isn't a STABLE 
branch for 5.x yet.
 
Can someone help?
 
Thanks